Position measuring devices for measuring the relative position of two objects are used particularly in connection with processing machines for measuring the relative position of a tool with respect to a workpiece which is to be processed.
European Patent Publication EP 0 151 002 B1 discloses a magnetic position measuring device for measuring the relative position of two objects. In particular, a periodic graduation is scanned by a scanning unit having two groups of magneto-resistive elements, each group having four magneto-resistive elements for generating zero-symmetrical output signals. The four magneto-resistive elements of the two groups which have output signals with phase positions of 0.degree. and 180.degree. are connected in series to form a first half-bridge circuit. The magneto-resistive elements of the two groups which have output signals with phase positions of 90.degree. and 270.degree. are connected in series to form a second half-bridge circuit. Two zero-symmetrical output signals with a mutual phase offset of 90.degree. are present at the center pick-ups of the two half-bridge circuits to obtain measured position values in a downstream-connected evaluation device.
In measuring devices of this type, the signal amplitude of the output signals is heavily dependent on the distance between the graduation and the scanning unit. Changes in the distance between the scanning unit and the graduation cause changes in the amplitude of the output signals which makes subsequent interpolation of the output signals more difficult.